Originally Posted by blowinstink
I think you want to add RPMs. If I run at 1800 RPM's (my sweet spot) I get @80% power (approx 5kn) and 2hrs/gal. If I run all out at 2800 RPMs I get 100% power (6kn+) but as little as 3/4hr/gal. You want to track your consumption levels. To do that, you'll need to keep an eye on your engine speed.
I guestimated about 2 litres per hour at 2000 RPM - that's about 1 gal(US) per two hours - pretty close.
What size engine do you have and what size boat? do you tow a dinghy?
I have a 30' Hunter (9500#'s approx.) and I used to tow a heavy dink. I just picked up a very light RIB (60 #'s) so mileage will go up. I have a 18 horse Yanmar diesel.